Complement system

Question 1

Classical vs. Alternate Pathway

Answer 1

Classical:

• must bind to antibody bound to antigen
• C1 binds an IgM or 2IgG, recruit C2 and C4
• form C2a4bC3 complex
• C3 binds and is cleaved
• *immune complex disorder if missing C2, C4 (Lupus)

Alternate:

• can bind directly to pathogen surface
• start with C3; factors B and D involved, C3b binds
• Factors I and H inactivate C3b to make iC3b. This prevents C3b all over the body and running out of C3b.
• Factor I deficiency –> C3 deficiency. makes you more susceptible to encapsulated bacteria like Staph/Strep

Question 2

DAF and MCP

Answer 2

inhibit alternate pathway activation on human cells. can only be activated on pathogen surfaces

Question 3

C3 receptors

Answer 3

when activated:

• stimulate phagocytosis
• clear immune complexes
• opsonization

-C3 targets C5. C3a and C5a increase vascular permeability –> inflammation

Question 4

Lytic/terminal pathway

Answer 4

C5-C9. C9 forms a pore in pathogen membrane to lyse

*need lytic pathway to get rid of Nesseria